Oh... just weed. Sorry, my mistake
I'm just a dumbass. Twice in my school district (once in my school and once in one nearby), the police put undercover agents in a lot of classes to catch drug dealers. I was quite surprise to hear there was no uproar about students' rights and such, except from a few radical teachers.
Lots of people do weed, but I don't see why athletes should be singled out - it's illegal for any student to be doing it, right? Why not impose the same restriction on all extracurricular activities, then you could say "Joining Chess Club is completely voluntary - if you don't want to pee in a cup, don't join!" Technically speaking, you wouldn't be denying anyone the right to public education, just excluding them from everything but going to class.